Personal emergency breathing systems have been proposed in the past and are now commercially available. One such system is described and illustrated in U.S. Pat. No. 5,315,987, the disclosure of which is incorporated herein by reference. Generally, in that system, a canister contains a protective hood, a multi-stage air purifying chemical filter attached to the hood and a valved mouthpiece complete with attached nose clip within the hood, all sealed within the canister until opened for use. When opened, the hood is drawn about an individual's head and neck, and ambient air is drawn into and through the filter with inhalation being effected through the mouthpiece. Exhaled air passes into the hood and outwardly between the individual's neck and the margin of the hood opening drawn about the neck. The multi-stage filter is designed to filter toxic gases, convert carbon monoxide to carbon dioxide and, as an alternative, oxygen, and provide filtered, breathable air for life support for a specified period of time, for example, in excess of ten minutes. This system has been found eminently useful for situations of one-time emergency use. For example, canisters of this type may be located in individual houses or apartment buildings for use by home owners or tenants in the event of a fire. In situations of that type, each user will typically open the canister, don the hood and breath filtered ambient air as he or she exits the house or building. Such system, however, is not designed for professional use, for example, by firefighters. Typically, firefighters will employ a supplied air respirator system comprised of a mask, usually a full-face mask, having a regulator and a hose connection to an air supply, for example, a tank carried on the back of the firefighter. When the firefighter dons the face mask and the system is operational, air is supplied from the tank to the firefighter via the hose, regulator and mask. The supply of respiratory air is, of course, limited. Should the professional, e.g., firefighter, use the entirety of the air supply or should the system malfunction while such professional remains in a hazardous area, i.e., an area having high concentrations of contaminants, smoke and the like, there can be great danger to life or health. A need, therefore, has arisen for a breathing system supplemental to a supplied air respiratory system for supplying breathable filtered air to the user in the event the air supply is exhausted or the latter system malfunctions.